Question:
At what point is it considered that one started modim (and thus has to go back to Retzei) is one forgot to say ya’aleh v’yavo? Once he said the full word or the first syllable?
Answer:
The poskim say that it is not considered considered starting modim until you say the first word, as half a eword is not considered starting. The same would apply to starting ato chonein, etc. if one forgot to say mashiv haruach or hamelech hakadosh.
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